with this philosopher. What is the cause of thunder?" Pliny thought he had explained it when he called it an earthquake in the air. Dr. Lister announced that lightning was caused by the sudden ignition of immense quantities of fine floating sulphur. Jonathan Edwards, in his diary of 1722, records the popular impression of the day upon this subject: "Lightning," he says, "seem to be an almost infinitely fine combustible matter, that floats in the air, that takes fire by sudden and mighty fermentation, that is some way promoted by the cool and moisture, and perhaps attraction of the clouds. By this sudden agitation, this fine floating matter is driven forth with a mighty force one way or other, whichever way it is directed, by the circumstances and temperature of the circumjacent air; for cold and heat, density and rarity, moisture and dryness, have almost an infinitely strong influence upon the fine particles of matter. This fluid matter thus projected, still fermenting to the same degree, divides the air as i